Because of the obviously missing T2 and the nice even number of schools in the top tier, law students, profs and anyone else who give a crap about this stuff add an imaginary T2 to the mix.

So, while TECHNICALLY the University of Louisville (Brandeis) is a tier 1 school, no one (except maybe U of L students themselves) would consider them that.

No it hasn't. In fact, when USNews first started ranking law schools (back in 1987) it only ranked 20 schools TOTAL. That's why a lot of people still refer to the top 20 (or top 14 sometimes) as the first tier.

As US News's rankings became more respected and more widely used, they expanded the number of schools that they rank to include regional schools, in addition to the 15-20 national schools.